Influence of Coupling Effect Between Recycled Brick Aggregate and Old Mortar on Mechanical Properties and Microscopic Damage Mechanism of Recycled Concrete

Recycled brick aggregate (RBA) and old mortar (OM) account for a large proportion of construction and demolition waste. To date, few systematic studies have addressed how their coupling interaction governs the performance of recycled concrete (RC). In this study, a two-factor experimental design was carried out, where RBA content (0–15%) and OM content (0–15%) were set as independent variables. The physical properties of recycled aggregates (RAs) and mechanical behaviors of RC were measured, and the underlying mechanisms were elaborated from both macroscopic and microscopic perspectives. Experimental results reveal that increasing RBA and OM contents raise the water absorption and crushing value of RA while reducing their apparent density. OM exerts a more substantial impact on water absorption and apparent density, whereas RBA predominantly controls the fluctuation of crushing value. When RBA dosage rises from 0% to 15%, the maximum reductions in compressive strength and flexural strength reach 20.7% and 23.4%, respectively. When OM dosage increases from 5% to 15%, the corresponding strength reductions are 24.8% and 20.3%. Scanning electron microscopy (SEM) characterizations prove that internal pores and microcracks within RBA, along with loose zones at the interface between OM and fresh mortar, act as vulnerable paths for crack initiation and propagation, which deteriorates the bulk macroscopic mechanical properties. Range analysis and analysis of variance confirm that RBA serves as the dominant influential factor. The contribution proportions of RBA to compressive strength and flexural strength are 68.3% and 72.5%, which exceed those of OM (31.7% and 27.5%). The combined action of RBA and OM produces an evident negative superposition effect, with no detectable performance compensation effect observed. This research lays a solid foundation for optimizing the mixing proportion of RA derived from construction waste and promoting the engineering application of RC.